Cassegrain antenna with dielectric guiding structure

ABSTRACT

An end-fire antenna having a thick-wall dielectric tube concentrically positioned about the principal axis of a cassegrain antenna and seated on the main reflector functions as a guiding structure for focusing the antenna radiation.

United States Patent Homer Eugene Bartlett Melbourne, Fla.

Mar. 27, I970 Oct. 5, 1971 The United States of America as representedby the Secretary of the Army Inventor Appl. No. Filed Patented AssigneeCASSEGRAIN ANTENNA WITH DIELECTRIC GUIDING STRUCTURE 5 Claims, 4 DrawingFigs.

U.S. Cl 343/755, 343/781, 343/785 Int. Cl 01g 19/10 Field of Search343/753, 574,755,785,840, 781

Die! uide casse rain fc'e'd 3 Primary ExaminerEli LiebermanAttorneys-Harry M. Saragovitz, Edward J. Kelly, Herbert Her] and MiltonW. Lee

ABSTRACT: An end-fire antenna having a thick-wall dielectric tubeconcentrically positioned about the principal axis of a cassegrainantenna and seated on the main reflector functions as a guidingstructure for focusing the antenna radiation.

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and ld- 42, H9 12 BACKGROUND OF THE INVENTION This invention relates todirectional antennas and more specifically to end-fire antennas of thethick-wall dielectrictube type.

End-fire antennas are well known in the art and encompass the class ofantennas having a principal axis and so designed and excited that themaximum intensity of radiation is directed along the principal axis.Typical antennas within the end-fife classification are the dielectricrod, disc on rod, yagi, and ferrite rod antennas.

A considerable amount of time and effort has been expended heretofore inattempts to develop an endf|re antenna which would be more easilytransported and would provide a specified required gain without havingto contend with the excessive length of the rod or the excessive size ofthe reflecting dish. Prior attempts to effect such an antenna had metwith little success until the present invention overcame the obstaclesand made feasible a highly transportable antenna array utilizing thickwall, end-fire, dielectric-tube antennas.

SUMMARY OF THE INVENTION One of the primary purposes of this inventionlies in the provision for a more easily transportable satellitecommunications antenna which is lightweight, compact and relativelyinexpensive.

The present invention successfully accomplishes the above purpose whilemaintaining all the advantages of the prior art without the attendantdisadvantages thereof. The improved results are accomplished byutilizing a thick wall dielectric tube as a guiding structure for acassegrain antenna.

BRIEF DESCRIPTION OF DRAWINGS The exact nature of this invention will bereadily apparent from consideration of the following specificationrelating to the annexed drawings in which:

FIG. 1 illustrates the geometry of the tapered wall tube;

FIG. 2 shows a cassegrain antenna used as the launcher for the end-fireantenna element;

FIG. 3 shows a thick-wall tube end-fire element with launcher; and,

FIG. 4 shows a tapered-wall tube end-fire element with launcher.

DESCRIPTION OF INVENTION The thick-wall dielectric-tube end-fire antennaevolved from research on cylindrical dielectric rod antennas, such asdisclosed by G. E. Mueller in U.S. Pat. Ser. No. 2,425,336. Generally,higher gains are obtainable with dielectric rod antennas than withconventional end-fire antennas and accordingly the first step inproviding the maximum gain of a dielectric rod antenna is to calculatethe optimum length and diameter of the rod.

The calculated diameter limitation for dielectric rods is expressed bythe formula:

A Lil where A designates wavelength and e refers to the dielectricconstant of the rod material. The optimum length of the dielectric rodis:

by the l-lansen-Woodyard criterion, a well-known principle of end-fireantennas.

It has been found that the optimum length of the dielectric rod is verynearly approximated by the formula L=A/s-l where the dielectric constantis low. Also, by experimental verification, the maximum diameter of therod for optimum performance has been found to be closely approximated byand therefore for maximum diameter rods the approximate length of theantenna would be: L-d=/). This formula provides for a very efi'ectivchighly transportable dielectric rod antenna until the required gainresults in excessive length for the rod antenna. The requirement L af/Aresults in impractical length of cylindrical rods where the diameter ofthe rod exceeds 1.5 feet at X-band. It has been found that a thick-walltube antenna and in particular the tapered-wall thick-wall tube antennahas definite size advantages over a dielectric rod of the same gain.From experimental results, the thick-wall tube appears to affordconsiderable length reduction and models with wall thicknesses ofapproximately one-third the tube outside diameter appears to have thelimitation:

and since for the models investigated. Therefore, the thick-walltubemust have a diameter of L3 times that of the rod for equivalentgain, which results in a length of about l8.5 percent that of the rod.

FIG. 1 shows the end-fire element geometry used to experimentallyoptimize the tapered-wall tube. The length, L, diameter, D, innerdimensions, d, and d,, inner flare angle, 0 and dielectric constant, s,were varied to determine the optimum tapered-wall tube shape. Severalmodels with various dielectric constants were investigated and effectiveapertures as high as 2.5 were attained.

As an example, the parameters of a tapered-wall tube with a dielectricconstant of 1.08 were optimized by varying 0 and L of FIGJ over wideranges to obtain the maximum effective aperture for a 16-inch diameter,D, tube. The dimension, d,,, was equal to zero. Angle 0 was varied fromzero degrees (a solid rod) to approximately l8". The maximum effectiveaperture was found to occur for an angle of l 1. The length was variedfrom 21 inches to 31 inches and the highest effective aperture wasobserved at a length of 26 inches. The maximum effective aperture wasfound to be 2.47 at 7.8 gc. and was broadband. In order to further.optimize the Fl .08 taperedwall tube, the angle 0 was varied on themodel by varying the dimension d, and holding d, constant at 2 inches.The best performance was obtained for an angle of approximately 95 wheremaximum effective aperture was 2.52, the highest obtained in the tests.

The optimum shape of a tapered-wall tube with dielectric constant of L2was also detennined. The length and diameter were calculated from theequations earlier presented. in these equations, letting A=l .5 and Fl.2, then L=8 inches and D=l0 inches. The dimension d, 'was fixed at linch and the angle 0 was allowed to vary by changing the dimension d,.The results indicate optimum performance at an angle of l6 compared to9.5 for the model with dielectric constant of [.08.

An analysis of the optimum shapes of the Fl .08 and Fl .2 modelsindicate that the design relations given by the formulae earlier setforth are indeed valid. Also, when compared to the complement of thecritical angle in the case of both Fl.08 and F1 .2 is approximately asfollows: 0,,,,,=0.66(6,,.).

in a transportable tapered-wall tube model, the end-fire elements couldbe fabricated of spaced discs of high dielectric constant material suchas Rexolite, polypropylene, polystyrene, Plexiglas, Styrofoam or any ofseveral other dielectric materials having a dielectric constant greaterthan one.

A launcher was designed to feed the optimized tapered-wall tube and tomatch its focal plane pattern. The launcher consisted of a dielectricguiding structure cassegrain feed and 16 inch paraboloidal reflectorwith the end-fire element concentrically mounted on the secondaryreflector about the principal axis thereof as shown in FIGS. 3 and 4.The particular launching arrangement of FIG. 2 is disclosed in U.S. Pat.Ser. No. 3,430,244 which issued to H. E. Bartlett et al. on 25 Feb.I969. The launcher utilized for the thick-wall tube tests is representedby the dual reflector (cassegrain) embodiment shown in FIG. 2. Theantenna system comprises a feed 2, subreflector 4 and a main reflectorl. The feed 2 may, for example, be a horn, placed along the axis ofreflectors l and 4. The location of the feed will depend almost entirelyon the shape of the subreflector surface. The dielectric guidingstructure 3 is arranged between the mouth of feed 2 and the convexsurface of subreflector 4, to guide substantially all of the energyradiated by the feed which would otherwise fall outside the subreflectorsurface (as ray OAX), toward the subreflector (as ray OAB). Almost allof the energy is thus directed toward the subreflector surface,reflected and passes through guide 3 at angles less than the criticalangle (as ray BCD). Upon striking reflector l, ray BCD is reflectedalong DE which is substantially parallel to the principal axis ofreflectors 1 and 4.

As shown in FIGS. 3 and 4, the thick-wall dielectric guiding structureis seated on main reflector l in concentric relationship with theprincipal axis in a manner whereby substantially all the rays emanatingfrom main reflector l in the direction of ray DE is captured by theguiding structure 5 and focused along the principal axis to form anend-fire directive antenna.

,As shown in previous calculations and experiments the use of thedielectric guiding structure 5 allows one to obtain improved resultswhile reducing the size of the reflecting dish required and showsremarkably improved results in comparison to a dielectric rod antenna tothe extent of reducing the length of the thick wall tube to aboutone-ninth the length of a solid rod of the same diameter.

FIGS. 3 and 4 disclose two separate embodiments of the invention in thecontext which it was designed to be used. lt is fairly obvious that theexact design or shape of the dielectric tube will vary according to onesspecific need as dictated by the earlier presented design formulas. Itshould also be noted here that the guiding formulas. 5 of FIGS. 3 and 4need not necessarily be of a circular configuration as previouslydescribed, but may take other forms and shapes.

The dielectric guiding structure 3 of FIGS. 3 and 4 should be designedwith scatter patterns having peaks approximately 40 off -axis and ahull-on-axis, which is generally the requirement for a launcher pattern.

In one particular instance a dielectric guiding structure cassegrainfeed was used having a 3.5-inch diameter choke horn with a having adielectric constant of L5 and an 8-inch diameter subreflector. Thedielectric guiding circuit flare angle was approximately 14 and the feedwas used with a l6-inch paraboloid main reflector.

It should be emphasized that the above disclosure was meant to beexemplary of the possible applications of this invention and not in anyway, a limitation thereof. Various modifications and changes in thespecific details of construction and operation described may be resortedto without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A tapered-wall, dielectric tube guiding structure mounted on the mainreflector of a cassegrain antenna wherein the feed and subreflector ofsaid antenna extends into the tapered-wall tube efiectively providing anend-fire antenna such that energy from the feed is effectively guidedalon and contained etween the walls of the tube and is directe along theaxial length of the guiding structure to produce at the opposite end ofthe tube a directive electromagnetic signal for transmission throughfree space, said tapered-wall dielectric tube has an inside taper withthe cross-sectional area progressively increasing along the axial lengthof the tube from the end of the guide seated relative to the feed towardthe opposite end of the tube from which the radiating signals emanateinto free space.

2. The guiding structure as set forth in claim I, wherein saidthick-wall dielectric tube is fabricated from a dielectric materialhaving a dielectric constant greater than one.

3. The guiding structure as set forth in claim 2, wherein the materialchosen for fabrication of the tube is selected from the group ofmaterials comprising, Rexolite, polypropylene, polystyrene; Plexiglasand Styrofoam.

4. The guiding structure as set forth in claim 1, wherein the outsidediameter of the tube is determined by applying the formula Ve-l where Ais the desired wavelength expressed in inches and e is the dielectricconstant of the material from which the tube is fabricated, thethickness of the wall is approximately one third the outside diameter ofthe tube and the length of the tube is approximately equal to (i /9A,where d is the outside diameter of the tube in inches and A is thedesired wavelength expressed in inches.

5. The guiding structure as set forth in claim 4, wherein the insidetaper of said dielectric tube is determined in accordance with theformula 0.66(0 where 0 defines the critical angle of the wall boundaryand is the angle of incidence of the electromagnetic waves generated bythe launcher on said boundary, above which total reflection of theincident wave occurs.

1. A tapered-wall, dielectric tube guiding structure mounted on the mainreflector of a cassegrain antenna wherein the feed and subreflector ofsaid antenna extends into the tapered-wall tube effectively providing anend-fire antenna such that energy from the feed is effectively guidedalong and contained between the walls of the tube and is directed alongthe axial length of the guiding structure to produce at the opposite endof the tube a directive electromagnetic signal for transmission throughfree space, said tapered-wall dielectric tube has an inside taper withthe cross-sectional area progressively increasing along the axial lengthof the tube from the end of the guide seated relative to the feed towardthe opposite end of the tube from which the radiating signals emanateinto free space.
 2. The guiding structure as set forth in claim 1,wherein said thick-wall dielectric tube is fabricated from a dielectricmaterial having a dielectric constant greater than one.
 3. The guidingstructure as set forth in claim 2, wherein the material chosen forfabrication of the tube is selected from the group of materialscomprising, Rexolite, polypropylene, polystyrene; Plexiglas andStyrofoam.
 4. The guiding structure as set forth in claim 1, wherein theoutside diameter of the tube is determined by applying the formula wherelambda is the desired wavelength expressed in inches and epsilon is thedielectric constant of the material from which the tube is fabricated,the thickness of the wall is approximately one third the outsidediameter of the tube and the length of the tube is approximately equalto d2/9 lambda , where d is the outside diameter of the tube in inchesand lambda is the desired wavelength expressed in inches.
 5. The guidingstructure as set forth in claim 4, wherein the inside taper of saiddielectric tube is determined in accordance with the formula 0.66(90* -theta cr) where theta cr defines the critical angle of the wall boundaryand is the angle of incidence of the electromagnetic waves generated bythe launcher on said boundary, above which total reflection of theincident wave occurs.